a) Field of the Invention
This invention relates to an engine-brake assisting system which makes it possible to obtain large engine brake force by opening and closing an exhaust valve of an engine at a timing different from that employed during a normal operation.
b) Description of the Related Art
As one of engine brake systems, an engine-brake assisting system has already been developed and commercialized. According to this engine-brake assisting system, an exhaust valve is opened and closed at a timing different from a normal exhausting timing when an accelerator is off, whereby a state of pressure within a cylinder is controlled to increase the ability of engine brake.
Such engine-brake assisting systems have been applied primarily to heavy vehicles such as large trucks and buses. In particular, such an engine-brake assisting system is used in combination with an exhaust brake system to produce strong engine brake force when an accelerator is off, thereby making it possible to obtain large braking force while reducing the load on a service brake.
A brief description will now be made of the operation principle of the above-mentioned engine-brake assisting system. During operation of this brake system, an intake valve and an exhaust valve are opened and closed, for example, in a manner described hereinafter.
In an intake stroke, the intake valve is opened as usual so that inducted air is introduced. In a compression stroke, both the intake valve and the exhaust valve are closed also as in an ordinary operation, and the inducted air within the cylinder is compressed.
Next, immediately before advancing from the compression stroke to an expansion stroke, the exhaust valve is opened to exhaust the compressed inducted air into an exhaust port by way of the exhaust valve. Repulsive force of the inducted air, which has been compressed in the compression stroke, no longer acts on a piston so that in the expansion stroke, no force acts in such a direction as pushing down the piston.
Further, after the exhaustion of the compressed air, the exhaust valve is closed near a top dead center to maintain the cylinder in a closed state during the expansion stroke. As a consequence, force is produced in such a way as preventing the piston from moving downward, resulting in application of engine brake force.
When the piston next reaches near a bottom dead center and the engine advances to an exhaust stroke, the exhaust valve is opened as usual to bring the internal pressure of the cylinder close to atmospheric pressure. When the piston subsequently reaches near the top dead center, an intake stroke is started again.
By repeatedly conducting such operation, brake forces in compression strokes and expansion strokes successively act on the piston, whereby the ability of engine brake is significantly increased. In other words, the engine is caused to perform pumping operation as negative work so that kinetic energy of a vehicle is absorbed and converted into braking force.
Incidentally, fuel injection is stopped during operation of such an engine-brake assisting system.
One example of such an engine-brake assisting system as described above is disclosed, for example, in Japanese Patent Application Laid-Open (Kokai) No. SHO 60-252113. With reference to FIG. 6 to FIG. 8, a brief description will now be made about the specific construction of such an engine-brake assisting system. A valve system of this engine is provided with an OHC valve train having a camshaft arranged on a cylinder head. Each cylinder is provided with intake valves 142,143 and exhaust valves 144,145.
A valve bridge (which may also be called an "intake crosshead") 146 is arranged over the intake valves 142,143, while another valve bridge (which may also be called an "exhaust crosshead") 147 is disposed over the exhaust valves 144,145.
Over these valve bridges 146,147, an intake rocker arm 149 and an exhaust rocker arm 150, both rockably supported on a rocker shaft 148, are arranged respectively in such a way that these arms are maintained at one ends thereof in contact with the corresponding bridges. During a normal operation of the engine, the intake valves 142,143 and the exhaust valves 144,145 are opened and closed in accordance with operation of the corresponding rocker arms 149,150.
The camshaft, which is designated at numeral 151, is provided with an intake cam 152 and an exhaust cam 153. The intake cam 152 and exhaust cam 153 are formed in such cam profiles as making the respective rocker arms 149,150 operate at a timing suited for the normal operation.
Further, as is illustrated in FIG. 6, a cylinder housing 111, as an essential element of the engine-brake assisting system, is arranged over the cylinder head, extending across the rocker shaft 148. Formed integrally with this cylinder housing 111 are a master cylinder 112, a slave cylinder 113, and a high-pressure fluid line (fluid line) 116 (FIG. 8) connecting the master cylinder 112 and slave cylinder 113 in communication with each other.
On the camshaft 151, an engine-brake assisting cam 138 is also arranged in addition to the above-mentioned intake and exhaust cams 152,153. By this engine-brake assisting cam 138, a master piston 125 disposed within the master cylinder 112 is reciprocally driven. Incidentally, the engine-brake assisting cam 138 is formed in such a cam profile that it drives the master piston 125 when the piston of the engine is located near the top dead center in a compression stroke.
On the other hand, a slave piston 129 is inserted within the slave cylinder 113 as depicted in FIG. 8. When working fluid is supplied through the high-pressure fluid line 116, the slave piston 129 is therefore driven responsive to operation of the master piston 125.
As is illustrated in FIG. 6 and FIG. 8, a piston rod 130 is also arranged underneath the slave piston 129. A lower end of this piston rod 130 is in contact with an upper end of the exhaust valve 145. Accordingly, when the slave piston 129 moves downward, the exhaust valve 145 is opened by way of the piston rod 130 irrespective of a state of operation of the exhaust rocker arm 150.
A directional control valve (solenoid valve) 114 is also arranged inside the cylinder housing 111 as shown in FIG. 6 and FIG. 8. As is illustrated in FIG. 8, control of this solenoid valve 114 makes it possible to change over the communication mode between two modes, one being a mode in which a working fluid supply line 136 and the high-pressure fluid line 116 are connected in communication with each other through a passage 35, and the other mode in which the high-pressure fluid line 116 and a working fluid return line 137 are connected in communication with each other through the passage 35.
Upon operation of the engine-brake assisting system of such a construction, fuel injection by an unillustrated fuel injection valve is stopped, and the solenoid valve 114 is changed over to connect the working fluid supply line 136 and the high-pressure fluid line 116 in communication with each other.
As a result, the high-pressure fluid line 116 is filled up with high-pressure working fluid. In this case, driving of the master piston 125 by the engine-brake assisting cam 138 leads to driving of the slave piston 129 by way of the high-pressure working fluid, whereby the exhaust valve 145 is opened near the top dead center in a compression stroke.
Accordingly, immediately before an advancement of the engine from a compression stroke to an expansion stroke, the exhaust valve 145 is opened and the compressed inducted air is exhausted by way of the exhaust valve 145. The repulsive force of the inducted air, which has been compressed in the compression stroke, therefore no longer acts on the piston, so that no force acts in such a direction as pushing down the piston in the expansion stroke.
Since the exhaust valve 145 is closed subsequent to the exhaustion of the compressed air, the inside of the cylinder is brought into a closed state in the expansion stroke. As a consequence, force is produced in such a way as preventing the piston from moving downward, resulting in application of engine brake force.
In such a conventional engine-brake assisting system, the housing 111 is, however, formed as an integral unit. The system hence becomes large as a whole, leading to a problem that the overall height of an engine becomes great.
Especially during operation of such an engine-brake assisting system as mentioned above, a large load is exerted on each member. The housing 111 is therefore required to have strength sufficient to withstand such large loads. From this requirement, the housing 111 also becomes large, resulting in another problem that the overall weight of the system increases.
According to the technique as described above, each cylinder is provided with its own solenoid valve 114 for changing over the engine-brake assisting system between an operation mode and a non-operation mode. Corollary to this, solenoid valves 114 are needed as many as cylinders, resulting in a further problem that the manufacturing cost is increased.
Moreover, each cylinder has to be provided with its own working fluid supply line 136 and working fluid return line 137, leading to a still further problem that the working manhour is increased and the manufacturing cost is also increased accordingly.